Conducting polymers (CP's) have received considerable attention in recent years due to their potential applications in a variety of electronic devices. CP's are presently used in commercial products, such as anti-static coatings on plastics, photographic film and electronic packaging materials. Other CP applications include solid electrode capacitors, through-hole plating of printed circuit boards, coatings for cathode ray tubes (to prevent dust attraction), hole injecting layers on indium tin oxide (ITO) substrates for electroluminescent devices, and sensors. Future applications, such as an ITO replacement leading to completely flexible, organic electronic devices, will require improvement in conductivity without sacrificing other properties such as optical transparency.
A variety of conducting polymers are described in the art, and several are commercially available, such as Baytron®P (product of Bayer AG) and Panipol® (product of Panipol Ltd.). Of the different CP families, (i.e., polyacetylenes, polyphenylenes, poly(p-phenylenevinylene)s, polypyrroles, polyanilines, and polythiophenes) polythiophenes are arguably the most stable thermally and electronically [see, “Handbook of Oligio- and Polythiophenes”, D. Fichou, Editor, Wiley-VCH, New York (1999), J. Roncali, Chem. Rev., 97, 173 (1997), A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., 110, 416 (1998), J. Roncali, J Mater. Chem., 9, 1875 (1999), J. Roncali, Annu. Rep. Prog. Chem. Sec. C., 95, 47 (1999), A. J. Heeger, Synth. Met., 55–57, 3471 (1993) and G. Kobmehl and G. Schopf, Adv. Polym. Sci., 129, 1 (1996)]. The Baytron®P product is a polythiophene/polystyrene sulfonate composition available as an aqueous dispersion containing about 1.3% solids. This aqueous dispersion is typically used to prepare coatings on various substrates. Baytron®P is prepared from 3,4-ethylenedioxythiophene (EDT) in aqueous or predominately aqueous media in the presence of polystyrene sulfonic acid (PSS, a dopant) using an oxidant such as iron trichloride [see, L. B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik and J. R. Reynolds, Adv. Mater., 12(7), 481 (2000)]. Coatings derived from Baytron®P have been reported to exhibit a wide range of surface resistance depending upon coating thickness. The surface conductivity of Baytron®P, and that of other CP based coatings, will increase with increasing coating thickness while the optical transmission will decrease. In most coating applications, the coatings must exhibit a useable combination of electrical conductivity, optical transparency and environmental stability (i.e., stability to moisture and oxygen). The appropriate balance of these properties is of critical importance; thus a means for improving this balance of properties would represent a significant advancement and enable new applications for these materials.
One approach to improving the electrical conductivity of polythiophenes has been via the use of additives (see e.g., U.S. Pat. No. 5,766,515). It has been shown that certain additives, when mixed with Baytron®P aqueous dispersion and subsequently used to make coatings, can produce an increase in the electrical conductivity (i.e., decrease in surface resistivity); however a high temperature treatment (at about 200° C.) is required to realize this improvement. The high temperature treatment is a major disadvantage because certain substrates cannot tolerate such high temperatures.
No explanation of the mechanism associated with conductivity enhancement is offered in the art; thus it is impossible to elucidate what additives may bring about this increase in electrical conductivity. Although the use of D-sorbitol (a sugar derivative) as an additive to Baytron®P is shown in the art as affecting electrical conductivity improvement, the enhancement in conductivity is only realized after shock drying at 200° C. for at least 90 seconds. The reported measured decrease in surface resistance (in the best case) is from about 3500 ohm/square to about 100 ohm/square. Because the coating thickness information for such coatings is not given, the volume resistivity of these coatings cannot be calculated. The high temperature shock drying at 200° C. step limits the use of the additive technology described in the art, as this temperature may exceed the thermal capability or cause deleterious shrinkage of certain plastic substrates. Further, the effects of this additive technology on optical transparency are not available.